1. Field of the Invention
The present invention relates to a color-image reader for optically and electronically sensing and reading a color image, which is recorded on a suitable recording medium, such as a transparency, a sheet of paper or the like.
2. Description of the Related Art
Such a color-image reader per se is well known, and is used, for example, in peripheral equipment associated with an image-processing computer for retrieving a color image. As a general representation, a color-image reader includes a solid-state line image sensor, such as a CCD (charge-coupled device) image sensor, a suitable light source for cyclically and successively illuminating the recording medium with primary color light rays: red-light rays, green-light rays and blue-light rays.
The CCD line image sensor includes a plurality of CCD elements aligned with each other, and each of the CCD elements generates and accumulates an electric charge in accordance with a received amount of monochromatic light rays (red, green, blue). As is well known, the CCD line image sensor possesses an electronic shutter function, and a time of electric-charge-accumulation or a time of exposure may be suitably controlled and regulated by using the electronic shutter function. As long as the CCD line image sensor is exposed to the monochromatic light rays, a degree of electric charge in each of the CCD elements is gradually increased, and the CCD elements finally reach saturation with the accumulated electric charges.
In operation, the recording medium is intermittently moved with respect to the CCD line image sensor such that the recording medium is scanned in a step-by-step manner with the CCD line image sensor. During each stoppage of the recording medium when being intermittently moved, the recording medium is subjected to one cycle of the successive emissions of the primary colors of light from the light source during each standstill of the transparent object, and the CCD line image sensor is successively exposed to the primary color light rays, passing through or reflected by the recording medium.
During the exposure of the CCD line image sensor to the primary color light rays, three single-lines of monochromatic image-pixel signals are successively outputted from the CCD line image sensor. After the outputting of the three single-lines of monochromatic image-signals from the CCD line image sensor, the recording medium is moved with respect to the CCD line image sensor by one scan-pitch. Thus, when the above-mentioned scanning operation is completed, three frames of monochromatic image-pixel signals, corresponding to the primary colors, can be obtained and used to reproduce the recorded color image of the recording medium, for example, on a TV monitor.
During the reading of the recorded color image from the recording medium, the exposure period, over which the CCD line image sensor is exposed to the monochromatic light rays, must be optimally regulated before the read color image can be obtained with the best contrast. Also, in order to reproduce the read color image with the best color balance, the three-frames of monochromatic image-pixel signals must be subjected to optimal color correction. Especially, as is well known when the recording medium is a transparency film, the color correction is critical, because a film material per se of the transparency film is colored.
An optimal exposure period is varied in accordance with a change in transparency of a recorded color image due to the reading of another recording medium. Accordingly, the optimal exposure period must be determined in accordance with the transparency of the recording medium. Conventionally, prior to a regular scanning operation for sensing and reading the recorded color image from the recording medium, a pre-scanning operation is carried out in order to determine an optimal exposure period with respect to the recorded color image of the recording medium concerned.
Nevertheless, conventionally, it is impossible to accurately determine the optimum exposure period, because a method for determining the optimum exposure period is based on an inaccurate assumption that there is a directly linear relationship between a time of exposure and a degree of electric charge accumulation in the CCD line image sensor, as discussed hereinafter in detail.
On the other hand, conventionally, color-correction parameters necessary for the color correction are determined on the basis of the three frames of monochromatic image-pixel signals obtained by the above-mentioned pre-scanning operation. Nevertheless, the determination of the color-correction parameters also cannot be accurately performed, because the three-frames of monochromatic image-pixel signals, obtained by the pre-scanning operation, do not properly represent color characteristics of the recorded color image of the recording medium. Also, an accurate determination of the color-correction parameter for a negative color transparency film is especially difficult, because the negative color transparency exhibits a wider exposure latitude than that of a positive color transparency.
Before accurate color-correction parameters can be obtained, the determination of the color-correction parameters should be based on three respective frames of monochromatic image-pixel signals derived from optimum exposure periods. Nevertheless, it is impossible to accurately determine the optimum exposure period with the conventional method as previously mentioned.